Diesel fuel may be delivered to customers with different properties at different times of the year. For example, additives may be mixed with diesel fuel to improve combustion during cold or warm weather. Further, different fuel refiners may process diesel fuel in slightly different ways so that diesel fuel properties may vary slightly from distributor to distributor. One property that may vary from season to season and distributor to distributor is a cetane number of the diesel fuel. A diesel fuel with a higher cetane number may advance phase of combustion (e.g., time of ignition relative to crankshaft position) in an engine while diesel fuel with a lower cetane number may retard phase of combustion in the engine. Changes in combustion phase can increase engine emissions such as HC, CO, NOx, fuel consumption, combustion noise, and/or carbonaceous particulate matter. Therefore, it may be desirable to compensate for fuels having cetane numbers that vary from fuels that have nominal cetane numbers. It may be possible to compensate for fuels having different cetane numbers by adjusting start of injection timing; however, simply adjusting start of injection timing can increase engine hydrocarbon emissions and particulate matter.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for operating an engine, comprising: combusting a first fuel in a cylinder, the first fuel mixture ignited via compression ignition; combusting a second fuel in the cylinder, a combustion phase of the cylinder advanced when the first fuel is combusted compared to when the second fuel is combusted; and adjusting a number of fuel injections provided to the cylinder during a cycle of the cylinder in response to the combustion phase.
By changing a number of injections delivered to a cylinder during a cycle of the cylinder or the relative amounts of fuel in each injection, it may be possible to compensate for changes in cetane that affect combustion phase of a cylinder. For example, during combustion of a nominal cetane fuel, three injections of fuel may provide desirable amounts of cylinder emissions and combustion noise. However, if a fuel is combusted in the cylinder that has a lower cetane than the nominal cetane fuel, the number of fuel injections provided to the cylinder during a cycle of the cylinder may be adjusted (e.g., increased) to compensate for a change in ignition dwell time that is related to combusting fuel with a lower cetane number. In other examples, fuel amounts may be exchanged between fuel pulses that are delivered to a cylinder to compensate for a change in fuel cetane.
The present description may provide several advantages. Specifically, the approach may reduce engine emissions when fuels having different cetane numbers are combusted by the engine. In addition, the approach may also be useful to reduce engine noise by controlling the rate of heat release during a cycle of the cylinder. Further, the approach may account for limitations of fuel injectors when fuel amounts are exchanged between different fuel pulses delivered to an engine cylinder.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.